Bright and dark fringes between two slits - waves

Google for youngs double slit experiment :)

 

Yep, owls is absolutely right. Here is an interesting link I found
( I don't have 15 posts yet thus I can't copy paste the link )
google for "young's double slit experiment" and click on the second link

If you still need help on this one, let us know

 

The two slits act as coherent sources of light, and at different points on the screen, the path difference between the two waves falling on that point varies, thus somewhere they interfere constructively (bright point, maxima) and at some points they interfere destructively (minima, dark band).

lambda / d = x/ L
d is distance between slits, lambda is the wavelength of monochromatic light, x is distance of first bright fringe from central maxima, L is distance of screen from the slits.
a bright fringe is obtained where the path difference between the light from the two slits is an integral multiple of lambda. so where
path diff = n.lamda .. bright fringe is obtained.
and when path difference is (2n-1).lambda/2 .......a dark band is obtained.